<p>请设计一个函数判断一棵二叉树是否 <strong>轴对称</strong> 。</p>

<p>&nbsp;</p>

<p><strong>示例 1：</strong></p>

<p><img alt="" src="https://pic.leetcode.cn/1694689008-JaaRdV-%E8%BD%B4%E5%AF%B9%E7%A7%B0%E4%BA%8C%E5%8F%89%E6%A0%911.png" /></p>

<pre>
<strong>输入：</strong>root = [6,7,7,8,9,9,8]
<strong>输出：</strong>true
<strong>解释：</strong>从图中可看出树是轴对称的。</pre>

<p><strong>示例 2：</strong></p>

<p><img alt="" src="https://pic.leetcode.cn/1694689054-vENzHe-%E8%BD%B4%E5%AF%B9%E7%A7%B0%E4%BA%8C%E5%8F%89%E6%A0%912.png" /></p>

<pre>
<strong>输入：</strong>root = [1,2,2,null,3,null,3]
<strong>输出：</strong>false
<strong>解释：</strong>从图中可看出最后一层的节点不对称。</pre>

<p>&nbsp;</p>

<p><strong>提示：</strong></p>

<p><code>0 &lt;= 节点个数 &lt;= 1000</code></p>

<p>注意：本题与主站 101 题相同：<a href="https://leetcode-cn.com/problems/symmetric-tree/">https://leetcode-cn.com/problems/symmetric-tree/</a></p>

<p>&nbsp;</p>
